I'm currently using a Vertex VXA-300 and have a Pilot-USA headset:

http://www.pilot-usa.com/pa2011.htm

While transmitting, there is pretty bad feedback (high pitched squeal)
in the headset. After doing some searching and trying a couple of
other headsets, it seems that the problem is that the radio is feeding
the transmission signal back into the cables and causing feedback.

Another member of our soaring club has the same problem with his Icom
using a Telex headset.

After doing quite a bit of research it seems that this is a common
problem with handhelds and headsets:

http://www.avionicswest.com/articles/a23.htm

We've both narrowed it down to the interference from the antenna but
cannot find a sensible way to solve the problem other than to
physically wrap the headset adapter cable around the antenna! This
solves the problem for both of us.

I contacted Vertex and they tell me that this will reduce the
transmission capabilities of the radio. They want me to send the radio
in and have it diagnosed saying that it may be a fault in the headset
adapter or radio, but I find it difficult to think that two different
people with two different radios and two different types of headsets
could have the exact same problem and resolve it in the same manner.
It also concerns me that this is a common problem and nowhere do any of
the handheld manufacturers mention it.

Just curious if anyone has any information or possible solutions.

Thanks,
Steve